Purification of lead and alloys thereof



Patented Dec. 12,- 19.3

UNITED STATES rnamcs'non or man AND armors manor Gustave E. Behr, NewYork, N. Y., assignorto National Lead Company, New York, N. Y acorporation of New Jersey 1 No Drawing. Application November 10, 1938,Serial No. 239,773

w '3 Claims. The present invention relatu; to lead and lead "alloys andhas for its primary object, among' others, a simple and efilcient meansfor removing zinc from lead and lead alloys.

Lead and lead alloys which are contaminated with zinc are principallyderived from two sources; (1) de-silvered Ilead bullion, and (2)secondary, or scrap lead and lead alloys which are returned to the tradefor refining. From the first source the presence of the zinc in the leadI bullion results from the use of zinc as a de-silverizing agent; fromthe second source, the zinc apparently results --tlon 1 I have foundthat if lead orlead alloys containing zinc are melted and treated undercontrolled conditions hereinafter to be described with ammonium chloridethe zinc is substantially wholly converted into zinc chloride whichrises to the top of the molten lead or lead alloy and may easily beremoved. Because of the volatility of ammonia and ammonium chloride nodeleterious addition of occurs.

I am aware that it has been proposed to remove monial lead except,perhaps, in such small amounts as to render removal unnecessary. Thezinc which occurs in de-silverized lead or sec ondary lead and leadalloys is not in the form of a zinc compound but as. the metal, eitherin solution in the lead or alloyed therewith. In such forms the zinccannot be removed by the mere physical means of dressing but a chemicalexpedient as described in my present invention must a be employed. i

According to the present invention, the lead or lead alloy to bepurified of zinc is melted. .The molten'metal is stirred, preferably'bymeans of a-stirrer which produces a good vortex and while maintainingthe metal at a temperature above thesolidifying point, powdered solidammonium chloride is gradually added. In a short time a thin liquid,more or less dark in color begins to collect on the surface of themolten metal. This liquid, I have found, is substantially'made up offrom unintentional contaminaf contaminants to the lead alloy doesnot'ordinarily occur in lead bullion or anti-.

liquid zinc chloride. Addition of ammonium chloride is continued untilno more liquid forms. The purified metal may be separated by anysuitable means: it may be' drained from the bottom of the kettle, theliquid may be drained off the metal or the whole may be allowed tosolidify 'and' the salt cake of zinc chloride removed from the surfaceof the solid metal. 4 a

The temperature at which I the ammonium chloride is added shouldpreferably be as low as possible to avoid excessive losses. of theammonium chloride by'volatilization but it should not be so low asunnecessarily to prolong the duration of the treatment. I have foundtemperatures between about 400 C. and about 475 C. are best adapted forthe practice of the invention.

The quantity of ammonium chloride required in order to remove zinc fromlead or lead alloys according to the invention will, of course, bedependent upon two factors: (1) e amount of zinc to be removed and (2)the loss of ammonium chloride by volatilization. Aside from that lost byvolatilization, at y leastsuflcient ammonium chloride should beintroduced into the molten .metal to convert the whole 01 the zincpresent into ammonium chloride, according to the equation: Zn+2NH4Cl-ZnClz+2NH3. 0n the whole, however, my experience has been that aboutfrom one and one-half to ten pounds ammonium chloride should be used forthe removal of one pound o'Lzinc. I Y

The addition of the ammonium chloride should,

preferably, although not necessarily, be made in 0 portions over aperiod of time in order to cut down as much aspossible volatilizationlosses and to permit observation of the efiect of each addition. Suchobservation enables the operator to determine whether or not thetreatment is finished. If no more liquid is formed after an addition ofammonium chloride the zinc has been substantially completely removed.

The following example illustrates but does not limit the invention:

trample 59,252 pounds of'a lead-tin alloy consisting o proximately of60% lead and 40% tin and c'ontaining 0.21% zinc was 'melted andagitated. 200 poundsot NHiCl was added over a periodof 40 minutes. Thetemperature during the operation was held at about 400 C. 335 poundssalt skim was recovered. Thejtreated alloy was en-. tirely free of zinc.

The foregoing descriptionhas been given for clearness of understandingand no undue limitatlons should be deduced therefrom but the appendedclaims should be read as broadly as possible in view of the prior art.

I claim:

1. A method for removing zinc from lead and lead alloys which comprisesmelting the metal to be purified and while agitating it at a temperatureabove the solidifying point thereof adding ammonium chloride thereto, inthe absence oi 10 added chlorine gas, and removing the zinc chlorideskim formed thereby.

2. A method for removing zinc from lead and lead alloys which comprisesmelting the metal to be purified and while agitating it at a temperaturebetween about 400 C. and about 475 C.

gradually adding thereto, in the absence of added cblorinmw. at least asuiiicient amount of ammonium chloride to convert the zinc contained

